Merge #1154

1154: Initial support for lang items (and str completion) r=flodiebold a=marcogroppo

This PR adds partial support for lang items.
For now, the only supported lang items are the ones that target an impl block.

Lang items are now resolved during type inference - this means that `str` completion now works.

Fixes #1139.

(thanks Florian Diebold for the help!)


Co-authored-by: Marco Groppo <marco.groppo@gmail.com>

crates/ra_syntax/src/ast/generated.rs

@@ -1325,6 +1325,7 @@ impl ToOwned for ImplBlock {
 
 
 impl ast::TypeParamsOwner for ImplBlock {}
+impl ast::AttrsOwner for ImplBlock {}
 impl ImplBlock {
     pub fn item_list(&self) -> Option<&ItemList> {
         super::child_opt(self)